1. Field of the Invention
The present invention relates to a fluoropolymer useful as an electrolyte material constituting an electrolyte membrane, or an electrolyte material contained in a catalyst layer for polymer electrolyte fuel cells.
2. Discussion of Background
Heretofore, for membranes for electrolysis of sodium chloride, for membranes or catalyst layers for polymer electrolyte fuel cells, it has been common to employ a polymer which is obtainable by hydrolyzing a copolymer of a fluoromonomer represented by the formula CF2═CF—(OCF2CFRx)x1—Ox2—(CF2)x3—SO2F (wherein Rx is a fluorine atom or a trifluoromethyl group, x1 is an integer of from 0 to 3, x2 is 0 or 1, and x3 is an integer of from 1 to 12, provided that x1+x2>0) with tetrafluoroethylene, or a polymer having sulfonic acid groups obtainable by further converting it to an acid form (hereinafter referred to as a sulfonic acid polymer).
Such a sulfonic acid polymer has a softening temperature in the vicinity of 80° C. Accordingly, the operation temperature of a fuel cell employing such a polymer is usually at most 80° C. However, in a case where hydrogen obtainable by reforming an organic compound such as methanol, natural gas or gasoline, is used as a fuel gas for a fuel cell, if carbon monoxide is contained even in a trace amount, the electrode catalysts will be poisoned, and the output of the fuel cell tends to be low. Accordingly, in order to prevent such a trouble, it is desired to increase the operation temperature. Further, also with a view to downsizing the cooling device for fuel cells, it is desired to increase the operation temperature, and preferably, a membrane for operation at a temperature of at least 120° C. is desired. However, the above-mentioned conventional sulfonic acid polymer has a low softening temperature and can not satisfy such demands.
As a polymer having a high softening temperature, a copolymer of a monomer represented by the following formula (y) (hereinafter referred to simply as a monomer (y)) with tetrafluoroethylene, has been proposed (WO03/037885). In the formula, QY is a fluorinated bivalent organic group, and each of Ry1 to Ry3 which are independent of one another, is a fluorine atom or a fluorinated monovalent organic group.

As mentioned above, fuel cells are preferably used under a high temperature condition. For example, polymer electrolyte fuel cells are preferably operated at a high temperature (for example, at a temperature of at least 120° C.) in order to facilitate heat removal and to increase the efficiency for power generation. For this purpose, a sulfonic acid polymer showing high mechanical strength in a high temperature region, is desired for e.g. the electrolyte membrane for polymer electrolyte fuel cells.
However, in a case where Qy in the monomer (y) in WO03/037885 is a perfluoro(etheric oxygen atom-containing alkylene) group such as —CF2OCF2CF2—, the softening temperature of the polymer obtainable by polymerizing the monomer (y) was not sufficiently high.
The present inventors have proposed a polymer which has a high softening temperature and has mechanical strength maintained even when used under high temperature conditions in WO05/096422. However, in a case where the polymer is produced by the method disclosed in WO05/096422, it was difficult to stabilize the polymer composition with high reproducibility.